End plate for a stack of fuel cells

ABSTRACT

The inventive end plate comprises pressure shield ( 21 ) and a supporting plate ( 30 ), whereby the forces, which serve to compress the stack ( 1 ) of fuel cells ( 2 ) and which are introduced via the force introduction locations ( 24 ), are introduced into the stack in the form of a compressive load in a defined and uniform manner.

This is the U.S. national stage of International applicationPCT/CH2004/000091, filed Feb. 19, 2004 designating the United States andclaiming priority to Swiss application 283/03, filed Feb. 23, 2003.These applications are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to an end plate for a stack of fuel cells,to a pressure plate and a bearing plate, from which an end plateaccording to the invention can be formed.

BACKGROUND OF THE INVENTION

Fuel cells or stacks of fuel cells are known. The individual fuel cellitself has a stacked structure comprising an electrolyte arrangedbetween its outer plates. There is an anode between the electrolyte andone outer plate and a cathode between the electrolyte and the otherouter plate. Solid and liquid electrolytes are known; the electrolytecan be held by a carrier structure or may itself have the requiredstrength to allow it to be installed in the fuel cell, depending onwhether a solid or liquid electrolyte is used. The operatingtemperatures also differ considerably, ranging from ambient temperatureto several hundred degrees C. and above.

In one of the many known designs to which the invention can be applied,a polymer electrolyte membrane (PEM) is used in the fuel cell. Inoperation, then, by way of example hydrogen in gas form is fed to theanode side of the fuel cell and, for example, oxygen-containing ambientair is fed to the cathode side of the fuel cell.

In the context of the present description, for the sake of simplicityall the reaction partners which participate in the chemical reaction ina fuel cell are referred to as fuels, as are their carrier fluids (e.g.ambient air as a carrier fluid for the reaction partner O₂). The fuelsare in fluid form. As has already been mentioned above, there arenumerous embodiments of fuel cells with a very wide range of fuels, andthe PEM fuel cell which is explained in more detail in the presentcontext serves merely as an example which helps to explain theconditions.

In general, structures which serve as passages for the fuels to passthrough are provided in the outer plates of the fuel cell, in such amanner that the electrolyte or membrane is covered with the fuels asuniformly as possible over the largest possible part of its area, whichleads to the desired electrolytic reaction and therefore to thegeneration of current.

The sealing concept of the fuel cell is crucial, since if the sealing isinadequate, the reaction of the fuels as they mix with one another willbe uncontrolled. In the case of the PEM fuel cell using hydrogen andoxygen as reaction partners, a leak leads to the detonating gasreaction. In general, special cord seals are used to reliably keep thefuels within their active regions. Of course, other sealing concepts arepossible.

The individual cell generates only a relatively low voltage; byconnecting the cells in series in a stack, it is possible to achieve avoltage which is adequate for the intended purpose and thereforesufficient power from the stack of fuel cells. Stacks of one hundred ormore fuel cells are usual. Nowadays, stacks of PEM fuel cells witharound a hundred cells, a power of 7 kw and a weight of approx. 20 kgare known.

In the stack, passages which run along the stack are used to supply theindividual fuel cells with the fuels. Special cooling is often alsoprovided, which likewise leads to passages for the coolant to flow tothe individual fuel cells. Moreover, cooling passages are then to beprovided in the individual fuel cell, and these in turn have to be keptsealed.

The result is that the routing of the media (fuels, coolants, etc.) inthe stack requires special design precautions as well as the sealingconcept. The passages for supplying the media are often integrated inthe outer plates of the individual fuel cell.

Finally, it should be taken into account that the series connection ofthe fuel cells in the stack leads to the flow of current which isgenerated by the stack flowing through the stack itself, i.e. from fuelcell to fuel cell, in each case through the outer plates thereof, whichare in contact directly or via intermediate layers.

Therefore, the contact resistance between the elements which are incontact with one another becomes critical for the power of the stack offuel cells: in the abovementioned, standard stack, there are in eachcase a hundred electrodes and outer plates, resulting in several hundredcontact surfaces with a corresponding contact resistance.

Fixing of a stack of fuel cells by clamping is in widespread use. Endplates provided at the ends of the stack are connected to one anothervia tie rods running along the stack and exert pressure on the stack,which holds the individual elements of the fuel cells and the fuel cellsthemselves in position in the stack.

The pressure required is considerable:

firstly, the fuels have to be passed through the fuel cell at a pressurewhich may quite easily be 2 to 3 bar.

Then, the seals have to be held under pressure, which likewise requiresa pressure of the abovementioned order of magnitude of another 2 to 3bar.

Finally, the contact resistance of the outer plates (generally graphiteplates) is directly dependent on the contact pressure, which leads tothe latter being very high.

The result of this is that in a conventional stack of fuel cells with 4to 6 tie rods, each tie rod introduces a force of 10⁴ N into the endplate, which leads to the required compressive load over thecross-sectional area of the stack of, for example, 100 to 300 cm².

This compressive load should be as uniform as possible, since themajority of the cross section of the stack, i.e. of the surface area ofthe outer plates or the membrane of the individual cell, is (has to be)available for routing the fuels for the electrochemical reaction wheresubstantially uniform conditions are present. Different requirements mayapply in the edge regions of the cell, e.g. at cutouts for the tie rodsor at the passages for routing the media to the individual cells.

As a result, a defined uniform compressive load is required over thecross section of the stack of fuel cells, as will be provided by theperson skilled in the art for the corresponding structural design of thecell. As has been mentioned, therefore, in the context of the invention,a “defined uniform compressive load” is to be understood as meaning aload which does change, including a load with sudden jumps in pressure,but on the proviso that the change in load or any sudden jumps inpressure are defined in a desired way by the person skilled in the art,so that every region in the fuel cell is subject to optimum pressure.Apart from special applications, however, it will be the case that onlyminor changes in load are desired and there will be no sudden jumps inpressure.

The clamping forces which are introduced into the end plate at the edgesides by the tie rods cause the end plate to bend with respect to itsbearing surface on the stack, with the result that the stack, as seen incross section, is subject to high compressive loads at the edge sidesand to only light compressive loads in the center, which contradicts thedesired, defined uniform loading.

Consequently, end plates are often designed as solid, heavy elementsreminiscent of armor plating. In particular a high weight and a highconsumption of material are undesirable if it is to be possible for thestack of fuel cells to be used in mobile applications, such as forexample vehicles, aircraft, etc., or if they are to be kept portable ingeneral.

The prior art has disclosed numerous embodiments of end plates by whichthe desired, defined uniform compressive loading of the stack of fuelcells can be achieved more successfully at a reduced weight ordeployment of material.

For example, WO 95/28010 shows a stack of fuel cells which arerectangular in cross section and having in each case four tie rodsacting on one set of opposite sides, while two brackets, the ends ofwhich are likewise subjected to load by tie rods, are provided on theopposite set of sides.

U.S. Pat. No. 6,428,921 shows a stack of fuel cells which arerectangular in cross section with tie rods running along the corners andacting on a double end plate. The outer plate has threaded bores intowhich bolts can be screwed so that they are then supported on the innerplate. Under operating load, the inner plate is prevented from bendingoutward, since screwing the bolts in sufficiently applies load in theinner region of the inner plate.

US 2002/0110722 shows an end plate with a set of springs arranged in theinner region on its side facing the stack; this set of springs exertscompressive load on the inner region of the stack even in the event ofbending of the end plate under the clamping forces acting via the tierods.

JP 9-259916 shows an embodiment with brackets which are subject to loadfrom the tie rods, run in the inner region over the end plate and act onthe end plate via local bearings.

The result of these embodiments is that the compressive load acts to anincreased degree not only in the edge regions of the stack but also inthe inner region. However, despite the conditions being improved, adefined uniform loading is scarcely achievable, and consequently locallyelevated compressive forces still have to be used in order to maintain aminimum pressure at the locations which are not subject to directcompressive loading.

U.S. Pat. No. 6,040,072 shows a double end plate, the outer plate ofwhich is thickened in the center but thinner at the edge sides, so thatit bears centrally against the inner plate. Under operating load of thetie rods, the result is deformation of the outer plate, in such a mannerthat it then bears against the entire surface facing the inner plate inone plane, resulting in an improved compressive loading of the stack.

It is known to calculate the change in thickness of the outer plate bynumerical methods with a view to achieving the desired, defined uniformcompressive loading. Modern machining methods allow sufficientlyaccurate reproduction of the nonuniformly curved surfaces of end platesof this type.

For example in the case of a stack which is rectangular in crosssection, the curvature may take place in one but preferably twodirections (corresponding to the sides of the stack).

However, a drawback which remains is still the high weight of an endplate of this type, since the thickness required remains considerable.In the abovementioned example of a stack of 20 kg, some 2 kg isattributable to the end plates, even though they are made from aluminum.The accurate machining of the surface contour is also complex, inparticular for series production.

A fin structure makes it possible to reduce the weight of an end plateof this type, but this further increases the outlay on machining.Numerical calculation models lead to structures which come very close tothe desired compressive loading of the stack with the weight reducedstill further. However, the outlay involved in producing structures ofthis type is huge.

Accordingly, the object of the present invention is to provide animproved end plate for a stack of fuel cells which allows a defineduniform compressive loading of the stack under operating load.

This object is achieved by the end plate, the pressure plate and thebearing plate described herein.

SUMMARY OF THE INVENTION

The present invention is directed towards an end plate for a stack offuel cells, having at least one force introduction location for forcesfor clamping the stack and a pressure shield, which extendssubstantially over its entire cross-sectional area, is operationallyconnected to the at least one force introduction location, and is curvedconvexly in at least one direction toward the stack as well as means forthe predetermined stabilizing of the shield convexity under load, andhaving further means, which are arranged on its convex side and can besupported on the stack, for transmitting defined, uniform pressureexerted by the shield to the adjoining stack.

The means for stabilizing the shield convexity may be designed as atension element which engages on the edge regions of the shield andstabilizes the edge regions at the predetermined distance from oneanother under the operating load of the shield.

The further means may have pressure fins, arranged perpendicular to theshield surface, for absorbing the operating load of the shield.

The end plate may have a plurality of force introduction locations forforces for clamping the stack which are arranged at the edge regions ofthe shield.

One force introduction location for forces for clamping the stack may beprovided, and the shield may be formed symmetrically or rotationallysymmetrically with respect to an axis of symmetry which runsperpendicularly through the end plate and coincides with a longitudinalaxis of the stack, the force introduction location may lie on the axisof symmetry, and the edge regions of the shield may run along theperiphery of the end plate and may be stabilized by a tension elementwhich acts on the edge regions and which may be designed as a clampingring.

The end plate may be of two-part design, comprising a pressure plate anda bearing plate, and the pressure plate may include the at least oneforce introduction location for the forces for clamping the stack aswell as the pressure shield, the pressure plate and bearing plate may bedesigned for operational interaction by nonpositive and/or positivelocking.

The end plate, which may be rectangular, for a stack of fuel cells whichis rectangular in cross section, may comprise a pressure shield that isconvex in only one direction and has surface lines which are parallel toone another and to two opposite edge regions of the end plate, thecorresponding edge regions of the end plate and of the shield may ineach case coincide and have the force introduction locations.

The stabilizing tension element may hereby connect the edge regions ofthe shield to one another and may preferably be designed as a tensionplate which extends over the entire length of the connected edgeregions.

The rectangular end plate may have a pressure plate that is designed asan extruded section, preferably of aluminum, which is substantiallyD-shaped in cross section.

The pressure shield which is curved convexly toward the stack allows theclamping forces which act at the force introduction locations to beintroduced in a defined uniform way into the bearing region. Thiseliminates the need for designs with transverse brackets, sets ofsprings, etc.; there is also no need for a solid, thick end plate ofconsiderable weight and entailing considerable manufacturing outlay formachining a specially curved surface which bears flat against the stackunder load. Over and above the object set, can the shield convexity canoptionally be selected in such a manner by the person skilled in theart, using known calculation methods, that the shield itself is subjectsubstantially only to compressive load by the reaction forces exerted bythe bearing region. Consequently, the demands imposed on the rigidity ofthe pressure shield are low, allowing it to be of correspondingly thindesign. It is in this way possible to produce a particularly lightweightend plate.

The means for the predetermined stabilization of the shield convexityunder load prevent the shield from widening or prevent deformation tothe shield convexity in such a manner as to produce a different load,e.g. a shear load, with the result that the shield region would thenhave to be correspondingly made thicker. According to the invention, theshield remains stable under the maximum load from the clamping forces,on the one hand, and the compressive load from the bearing region.

The bearing region acts as a transition element from the curved pressureshield to the next, planar element of the stack of fuel cells that is tobe fixed, and transmits only the desired compressive forces to thisstack.

An embodiment of a two-part end plate is particularly advantageous. Thepressure plate can be produced as an aluminum extruded section with aD-shaped cross section, with the curved part of the D corresponding tothe pressure shield and the straight back of the D corresponding to thetensioning plate. The weight of a structure of this type isadvantageously low; the use of extruded sections is expedient in seriesproduction. The bearing plate is then provided with a set of fins whichon one side adjoin the curve of the D as a negative and on the otherside, parallel to the back of the D, form the connection surface to thefirst element of the stack. Weight and production costs are just asfavorable as with the pressure plate.

The curvatures of the surfaces of shield and bearing plate whichinteract with one another as described above may differ in apredetermined way, with the active surface of the shield being curved toa greater extent that the corresponding surface of the bearing plate, insuch a manner that the active surface of the shield, under operatingload, can be widened in a predetermined way and can be pressed in adefined, uniform way against the entire corresponding surface of thebearing plate.

In such an embodiment, a gap may be provided between the interactingsurfaces of pressure shield and bearing plate, with the gap wideningoutward starting from a contact location. This allows a predetermineddeformation of the pressure shield, which assists with the defineduniform transmission of pressure from the pressure shield to the bearingplate. An optimization of this nature takes account of the fact that nocomponent is absolutely rigid, and consequently makes deliberate use ofthe elastic deformation which always occurs. The person skilled in theart will preferably design this gap in such a way that the pressureshield under operating load to a certain extent rolls into the bearingplate and then, in the load-deformed position, is only under compressiveload.

The invention is also directed to a pressure plate for a stack of fuelcells for operational interaction with a bearing plate, comprising atleast one force introduction location for forces for clamping the stackand a pressure shield, which extends substantially over its entiresurface, is operationally connected to the at least one forceintroduction location and is convexly curved in at least one directiontoward a bearing plate, as well as means for the predeterminedstabilization of the shield convexity under load.

Such a pressure plate can be combined with a bearing plate designed inany desired way, provided that the latter adjoins the pressure shield inthe form of the negative.

The present invention is also directed to a bearing plate for a pressureplate of a stack of fuel cells, comprising pressure fins for absorbingthe operating load of a pressure shield by way of the outer surfacesthereof bearing against the shield convexity, and a planar bearingsurface, which is on the opposite side remote from the outer sides ofthe pressure fins, to provide support against an adjoining element ofthe stack of fuel cells.

Such a bearing plate can be combined with a pressure plate of anydesired design, provided that the latter adjoins it in the style of thepositive.

Particular embodiments are described in more detail below with referenceto the figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically depicts a perspective view of a stack of fuelcells provided with end plates according to the invention,

FIG. 2 diagrammatically depicts a perspective view of a pressure plateaccording to the invention,

FIG. 3 diagrammatically depicts a perspective view of a bearing plateaccording to the invention,

FIG. 4 diagrammatically depicts a pressure plate according to theinvention with a plurality of segments,

FIG. 5 diagrammatically depicts a pressure plate according to thepresent invention curved in two directions, and

FIG. 6 diagrammatically depicts a rotationally symmetrical end platewith a corresponding bearing plate.

FIG. 7 diagrammatically depicts a perspective view of a stack of fuelcells provided with end plates according to the invention with a singlepiece design.

DESCRIPTION OF VARIOUS AND PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a rectangular stack 1 of fuel cells with rectangular endplates 10 and fuel cells 2 which are stacked on top of one another andare clamped together by means of the end plates 10. Tie rods 3 connectthe end plates 10 to one another and can be tensioned by way of screwthreads (not shown) with a tensioning member 4, so that the tie rods 3introduce the forces for clamping the stack 1 into the end plates.

A layout of this nature is fundamentally known.

According to the invention, the end plates 10 are of two-part design;there is in each case one pressure plate 20 and one bearing plate 30.

The pressure plate 20 has a pressure shield 21 and a tension element asmeans for the predetermined stabilization of the shield convexity underload, which is in this case designed as a tension plate 22. Pressureshield 21 and tension plate 22 are connected to one another at theirrespective, opposite edge regions 23. The force introduction locations24 of the tie rods 3 lie in the edge regions 23. The plane of thetension plate 22 intersects the pressure shield 21 in a straightintersection line 25 indicated by dot-dashed lines. The forceintroduction locations 24 preferably lie on or as close as possible tothe intersection lines 25, which minimizes the loading of the pressureplate 20 under operating load and allows the pressure plate 20 to be ofmaterial-saving design.

The pressure shield 21 is curved convexly toward the bearing plate inone direction (in this case in the transverse direction q of the stack1) and has parallel surface lines (running in the longitudinal direction1 of the stack 1), which for their part run parallel to the edge regions23. When seen in cross section, the pressure plate 20 has a D-shapedprofile.

A convex side 26 of the pressure shield, under operating load, bears inthe style of a negative form against the correspondingly designedbearing plate 30, while the tension plate 22 engages (in the edgeregions 23) on its concave side 27.

As a result of the convex shield curvature, the pressure shield 21exerts a defined, uniform compressive load on the bearing region 30,which is transmitted by the latter to a fuel cell 2 which adjoins it (orto another, adjoining element of the stack 1). Consequently, the fuelcells 2 are under the desired, defined uniform load. The tension plate22 stabilizes the shield convexity under load by acting on the edgeregions 23 of the pressure shield 21 and thereby fixing the pressureshield in place.

The bearing plate 30 has pressure fins 31 with outer surfaces 34, thepressure fins 31 projecting perpendicularly from the pressure shield 21and over their outer surfaces 34 forming the negative form against whichthe pressure shield 21 bears. Consequently, the pressure fins 31 absorbthe operating load of the pressure shield 21 and in the present case areconnected to one another by means of a baseplate 32; the latter in turnbears against the adjoining element or an adjoining fuel cell 2 of thestack 2. The baseplate 32 holds the pressure fins 31 in position, andthese fins in turn introduce the operating load from the pressure shield21 into the stack 1.

Media-carrying passages 33, which connect the supply passages for fuels,coolant, etc., which run along the stack 1 and are not shown in thefigure, to the outside world, are also illustrated. Supply passages forfeeding the individual fuel cells 2 which run along the stack 1 areknown. It is advantageous that the use of the bearing plate 30 accordingto the invention means that there is no need for a special element forconnecting the supply passages to the outside world.

The convexity of the pressure shield can be calculated by the personskilled in the art and is determined as a function of the desired,defined uniform compressive loading of the fuel cells 2. Since everystructure is elastically deformed under load, it is advantageous for thecurvature of the pressure shield 21 to be selected to be slightlygreater than the curvature of the outer surfaces 34 of the pressure fins31. Consequently, without operating load, the pressure shield 21 bearscentrally in the bearing surfaces 34, but under operating load comes tobear completely and in a defined uniform way against the outer surfaces34 of the pressure fins 31 by virtue of the fact that it can be widenedin a predetermined way under this operating load, with the result thatthe calculated compressive load distribution also occurs under realconditions. This deformation can likewise be calculated by the personskilled in the art, including the deformation to the tension plate 22.

In accordance with the curvature of the pressure shield 21 in only onedirection, the transverse direction q, there is no defined, uniformcompressive loading in the longitudinal direction 1. This drawback isonly apparently present:

firstly, the tie rods 3 can now be arranged offset from the corners ofthe stack 1 more toward the center of its longitudinal side, so that theundesired differences in load are reduced.

Furthermore, tie rods 3 can be provided in any desired number, andconsequently the differences in load can be reduced in a predeterminedway.

Finally, the dimensions of the bearing plate 30 can be adapted: underoperating load, applied at the force introduction locations 24, the endplate 30 is deformed in wavy fashion along the edge regions 23 thatinclude the force introduction locations 24. This deformation can becalculated and then the corresponding negative established. In otherwords, a negatively wavy bearing plate which is then deformed to beplanar under operating load is produced.

The result of the pressure shield which is curved in only one directionaccording to the invention is that the defined, uniform compressive loadis produced over the entire cross-sectional area of the stack 1.

For series production of a stack 1 of fuel cells, it is advantageous forthe pressure plate 20 to be produced from an extruded section. Thebearing plate 30 with its pressure fins 31 arranged perpendicular to thebaseplate 32 is likewise easy to produce. The end plate 10 which isthereby formed according to the invention has a low weight with lowproduction costs.

FIG. 2 shows a perspective view of a pressure plate 20 according to theinvention. The figure illustrates the force introduction locations 24for the tie rods 3, which engage by way of their tong-like grippers 5(FIG. 1), on correspondingly designed mating pieces 28 of the pressureplate 20. Mating pieces of this type can easily be formed on theextruded section produced in an extrusion process.

In operation, pressure plate 20 and bearing plate 30 are nonpositivelyconnected to one another; if the grippers 5 and the mating pieces 28 areof suitable design, the grippers laterally surround the bearing plate30, which in addition to the nonpositive lock also results in a positivelocking of the plates (20, 30) and connects the two plates to form anassembly, the end plate 10.

FIG. 3 shows a perspective view of a bearing plate 30 according to theinvention. The figure illustrates the baseplate 32 with the pressurefins 31 and their outer surfaces 34, against which the pressure shield21 comes to bear. The media-routing passages 33, which guide the medialaterally into the bearing plate 30 and then pass them downward at rightangles into the stack 1, are also illustrated.

FIG. 4 diagrammatically depicts a pressure plate according to theinvention with a plurality of segments. Transverse webs 29 divide thepressure shield 21 into a plurality of segments 21′. As a result, underoperating load, each segment 21′ is loaded in the same way as the shield21 (FIG. 2). As a result, separate cavities are formed in the pressureplate, which has the advantage that the pressure plate can be used, forexample, for routing the media. A further advantage of dividing thepressure shield 21 into segments 21′ is that the overall size of boththe pressure plate 20 and of the bearing plate 30 is reduced.

The use of transverse webs 29 leads, at the location of the connectionto the tension plate 22, to a kink 29′ in the latter, so that the forcesat the location of the connection cumulatively amount to zero, andconsequently the tension plate, as before, is still exposedsubstantially only to tensile forces.

FIG. 5 diagrammatically depicts a pressure plate 40 which is curved intwo directions, having a pressure shield 41 and a tension plate 42;consequently, a defined uniform compressive loading of the fuel cells 2can be achieved in the longitudinal direction l and in the transversedirection q. The tension plate 41 can be fixed on the pressure shield 41by conventional mechanical means, not illustrated in the figure, such asa screw connection or adhesive bonding, etc. The curvature of thepressure shield 41 can be determined numerically by the person skilledin the art.

FIG. 6 diagrammatically depicts a cross section through a rotationallysymmetrical end plate 50 in accordance with the present invention. Apressure shield 51 is of rotationally symmetrical design and has an axisof symmetry 52. The pressure shield 51 is subject to load by a singletie rod 53 lying on the axis of symmetry 52 at the location of the forceintroduction location 54. Means for the predetermined stabilization ofthe shield convexity are designed as a clamping ring 56 formedintegrally on the pressure shield 51.

Pressure fins 60 with bearing surfaces 61 for the pressure shieldintroduce the compressive loading, via a baseplate 62, into the adjacentfuel cells 2 of the stack 1.

The end plate 50 may comprise a pressure plate, comprising the pressureshield 51, the clamping ring 56 and the force introduction location 54,as well as a bearing plate having the pressure fins 60 and the baseplate62. The figure illustrates an integral design of the end plate 50; thebaseplate 62 may optionally then be omitted.

It can be seen from the figure that the axis of symmetry 52 coincideswith the axis of symmetry of the stacked fuel cells 2.

It is also possible for the end plate 10 (FIG. 1) to be of single-piecedesign, in which case the baseplate 32 can optionally likewise beomitted (FIG. 7). In this case, the length of the pressure fins 31between pressure shield 21 and adjoining element of the stack 1 isadvantageously formed in such a way that without operating load thepressure fins are only in contact with the center of the adjoiningelement of the stack 1, but are at a distance from it at the edge sides.This distance is dimensioned in such a way by the person skilled in theart that it drops to zero under operating load. As a result, underoperating load, the pressure fins come to bear flat against theadjoining element of the stack 1; the desired, defined uniformcompressive load is introduced into the stack 1.

1. An end plate for a stack of fuel cells comprising at least one forceintroduction location for forces for clamping the stack, a pressureshield, which extends substantially over the fuel cell stack's entirecross-sectional area, is operationally connected to the at least oneforce introduction location, and is curved convexly in at least onedirection toward the stack to form, in cross-section, a curved part of aD-shape, at least one predetermined stabilizing device forming thestraight back of said D-shape, wherein said device stabilizes convexityof the pressure shield under load and wherein said pressure shield andsaid at least one stabilizing device are connected to one another attheir respective, opposite edge regions to form said D-shape, andtransmitting devices arranged on a convex side of the pressure shieldand which can be supported on the stack, wherein said transmittingdevices transmit defined, uniform pressure exerted by the pressureshield to the adjoining stack.
 2. The end plate of claim 1, wherein theat least one stabilizing device is designed as a tension element whichengages on said opposite edge regions of the pressure shield andstabilizes the edge regions at a predetermined distance from one anotherunder an operating load of the pressure shield.
 3. The end plate ofclaim 2 for a stack of fuel cells which is rectangular in cross section,wherein the end plate is rectangular, and wherein the pressure shield isconvex in only one direction and wherein said one direction extendsparallel to two opposite edge regions of the end plate, thecorresponding edge regions of the end plate and of the shield in eachcase coinciding and having the force introduction locations.
 4. The endplate of claim 2, wherein the tension element is a tension plate.
 5. Theend plate of claim 1, having a plurality of force introduction locationsfor forces for clamping the stack, wherein the force introductionlocations are arranged at edge regions of the pressure shield.
 6. Theend plate of claim 1 being of two-part design and, comprising a pressureplate and a bearing plate, wherein the pressure plate includes the atleast one force introduction location for the forces for clamping thestack as well as the pressure shield, and wherein the pressure plate andbearing plate are designed for operational interaction by nonpositiveand/or positive locking.
 7. The end plate of claim 6, wherein thecurvatures of the surfaces of the shield and the bearing plate whichinteract with one another differ in a predetermined way, with an activesurface of the pressure shield being curved to a greater extent than thecorresponding surface of the bearing plate so that the active surface ofthe pressure shield, under operating load, can be widened in apredetermined way and can be pressed in a defined, uniform way againstthe entire corresponding surface of the bearing plate.
 8. The end plateof claim 6 for a stack of fuel cells which is rectangular in crosssection, wherein the end plate is rectangular, and wherein the pressureshield has a convex and a concave side and is convex in only onedirection and wherein said one direction extends parallel to twoopposite edge regions of the end plate, the corresponding edge regionsof the end plate and of the pressure shield in each case coinciding andhaving the force introduction locations.
 9. The end plate of claim 8,wherein the at least one stabilizing device is designed as a stabilizingtension element and wherein the stabilizing tension element connects theedge regions of the pressure shield to one another.
 10. The end plate ofclaim 9, wherein said stabilizing tension element is designed as atension plate which extends over the entire length of the connected edgeregions.
 11. The end plate of claim 6, wherein the bearing plate furthercomprises passages for carrying the media of a stack of fuel cells. 12.The end plate of claim 1, wherein the transmitting devices have passagesfor carrying media.
 13. The end plate of claim 1, wherein the pressureshield which is curved convexly in at least one direction toward thestack has a convex side towards the stack and a concave side away fromthe stack.
 14. An end plate for a stack of fuel cells comprising atleast one force introduction location for forces for clamping the stack,a pressure shield, which extends substantially over the fuel cellstack's entire cross-sectional area, is operationally connected to theat least one force introduction location, and is curved convexly in atleast one direction toward the stack, at least one predeterminedstabilizing device, wherein said device stabilizes convexity of thepressure shield under load, and having transmitting devices arranged ona convex side of the pressure shield and which can be supported on thestack, wherein said transmitting devices transmit defined, uniformpressure exerted by the pressure shield to the adjoining stack, and havepressure fins, arranged perpendicular to the pressure shield surface,for absorbing an operating load of the pressure shield.
 15. An end platefor a stack of fuel cells comprising at least one force introductionlocation for forces for clamping the stack, a pressure shield having aconvex side and a concave side, which extends substantially over thefuel cell stack's entire cross-sectional area, is operationallyconnected to the at least one force introduction location, and is curvedconvexly in at least one direction toward the stack, at least onepredetermined stabilizing device, wherein said device stabilizesconvexity of the pressure shield under load and wherein said pressureshield and said at least one stabilizing device are connected to oneanother at their respective, opposite edge regions, and transmittingdevices arranged on the pressure shield, wherein said transmittingdevices transmit defined, uniform pressure exerted by the pressureshield to the adjoining stack, wherein one force introduction locationfor forces for clamping the stack is provided, and wherein the pressureshield is rotationally symmetrical with respect to an axis of symmetrywhich runs perpendicularly through the end plate and coincides with alongitudinal axis of the stack, wherein the force introduction locationlies on the axis of symmetry, and edge regions of the shield run alongthe periphery of the end plate and are stabilized by a tension elementwhich acts on the edge regions and is designed as a clamping ring. 16.An end plate for a stack of fuel cells comprising at least one forceintroduction location for forces for clamping the stack which isrectangular in cross section, a pressure shield, which extendssubstantially over the fuel cell stack's entire cross-sectional area, isoperationally connected to the at least one force introduction location,and is curved convexly in at least one direction toward the stack, atleast one predetermined stabilizing device, wherein said devicestabilizes convexity of the pressure shield under load and wherein saidpressure shield and said at least one stabilizing device are connectedto one another at their respective, opposite edge regions, andtransmitting devices arranged on a convex side of the pressure shieldand which can be supported on the stack, wherein said transmittingdevices transmit defined, uniform pressure exerted by the pressureshield to the adjoining stack, wherein the at least one stabilizingdevice is designed as a tension element which engages on said oppositeedge regions of the pressure shield and stabilizes the edge regions at apredetermined distance from one another under an operating load of thepressure shield, wherein the end plate is rectangular, wherein thepressure shield is convex in only one direction and wherein said onedirection extends parallel to two opposite edge regions of the endplate, the corresponding edge regions of the end plate and of the shieldin each case coinciding and having the force introduction location(s),and wherein the endplate is of single piece design.
 17. The end plate ofclaim 16, wherein said stabilizing element is designed as a tensionplate which extends over the entire length of the connected edgeregions.
 18. An end plate for a stack of fuel cells comprising at leastone force introduction location for forces for clamping the stack whichis rectangular in cross section, a pressure plate, wherein the pressureplate is designed as an extruded section and is substantially D-shapedin cross section to provide a D-shape, with a curved part of the D-shapecorresponding to a pressure shield and a straight back of the D-shapecorresponding to a tension element wherein the pressure shield, whichextends substantially over the fuel cell stack's entire cross-sectionalarea, is operationally connected to the at least one force introductionlocation, and is curved convexly in at least one direction toward thestack, at least one predetermined stabilizing device, wherein saiddevice stabilizes convexity of the pressure shield under load andwherein said pressure shield and said at least one stabilizing deviceare connected to one another at their respective, opposite edge regions,and transmitting devices arranged on a convex side of the pressureshield and which can be supported on the stack, wherein saidtransmitting devices transmit defined, uniform pressure exerted by thepressure shield to the adjoining stack, wherein the at least onestabilizing device is designed as the tension element which engages onsaid opposite edge regions of the pressure shield and stabilizes theedge regions at a predetermined distance from one another under anoperating load of the pressure shield, wherein the end plate isrectangular, and wherein the pressure shield is convex in only onedirection and wherein said one direction extends parallel to twoopposite edge regions of the end plate, the corresponding edge regionsof the end plate and of the shield in each case coinciding and havingthe force introduction location(s).
 19. The end plate of claim 18,wherein the pressure plate is designed aluminum.
 20. An end plate for astack of fuel cells comprising at least one force introduction locationfor forces for clamping the stack, a pressure shield, which extendssubstantially over the fuel cell stack's entire cross-sectional area, isoperationally connected to the at least one force introduction location,is curved convexly in at least one direction toward the stack, and isdivided by integrally formed transverse webs into a plurality ofsegments, at least one predetermined stabilizing device, wherein saiddevice is a tension element and stabilizes a convexity of the pressureshield under load and wherein said integrally formed transverse websresult in (1) separate cavities within a pressure plate comprising thepressure shield and said tension element and (2) kinks in said tensionelement, so that, where the webs are connected to the tension element,forces cumulatively amount to zero, and transmitting devices arranged ona convex side of the pressure shield and which can be supported on thestack, wherein said transmitting devices transmit defined, uniformpressure exerted by the pressure shield to the adjoining stack.
 21. Theend plate of claim 20, wherein the tension element is a tension plate.22. An end plate for a stack of fuel cells comprising at least one forceintroduction location for forces for clamping the stack which isrectangular in cross section, a pressure shield, which extendssubstantially over the fuel cell stack's entire cross-sectional area, isoperationally connected to the at least one force introduction location,and is curved convexly in at least one direction toward the stack, atleast one predetermined stabilizing device, wherein said devicestabilizes convexity of the pressure shield under load and wherein saidpressure shield and said at least one stabilizing device are connectedto one another at their respective, opposite edge regions, andtransmitting devices arranged on a convex side of the pressure shieldand which can be supported on the stack, wherein said transmittingdevices transmit defined, uniform pressure exerted by the pressureshield to the adjoining stack, wherein the end-plate is of two-partdesign and, comprises a pressure plate and a bearing plate, wherein thepressure plate includes the at least one force introduction location forthe forces for clamping the stack as well as the pressure shield,wherein the pressure plate and bearing plate are designed foroperational interaction by nonpositive and/or positive locking, whereinthe end plate is rectangular, and wherein the pressure shield has aconvex and a concave side and is convex in only one direction andwherein said one direction extends parallel to two opposite edge regionsof the end plate, the corresponding edge regions of the end plate and ofthe pressure shield in each case coinciding and having the forceintroduction locations, wherein the at least one stabilizing device isdesigned as a stabilizing tension element and wherein the stabilizingtension element connects the edge regions of the pressure shield to oneanother, wherein said stabilizing tension element is designed as atension plate which extends over the entire length of the connected edgeregions, and wherein the pressure plate is designed as an extrudedsection, and is substantially D-shaped in cross section to provide aD-shape, with a curved part of the D-shape corresponding to the pressureshield and a straight back of the D-shape corresponding to the tensionplate.
 23. The end plate of claim 22, wherein the pressure plate isdesigned of aluminum.
 24. An end plate for a stack of fuel cells beingof two-part design and, comprising a pressure plate and a bearing plate,wherein the pressure plate includes the at least one force introductionlocation for the forces for clamping the stack as well as a pressureshield, and wherein the pressure plate and bearing plate are designedfor operational interaction by nonpositive and/or positive locking, andwherein the pressure shield extends substantially over the fuel cellstack's entire cross-sectional area, is operationally connected to theat least one force introduction location, and is curved convexly in atleast one direction toward the stack, at least one predeterminedstabilizing device, wherein said device stabilizes convexity of thepressure shield under load, and having transmitting devices arranged ona convex side of the pressure shield and which can be supported on thestack, wherein said transmitting devices transmit defined, uniformpressure exerted by the pressure shield to the adjoining stack, whereinthe bearing plate comprises pressure fins, wherein said pressure finsabsorb the operating load of the pressure shield by way of outersurfaces thereof bearing against the pressure shield convexity, and aplanar bearing surface, which is on the opposite side remote from theouter sides of the pressure fins, wherein said planar bearing surfaceprovides support against an adjoining element of the stack of fuelcells.